jimtcl: explicitly set --host / --build

Fixes:
http://autobuild.buildroot.net/results/d13/d137680bf1c0ebfacef635cb2f0fc14524759143/



Otherwise the (handwritten) configure script uses a config.guess script from
2010 to figure out the build host, breaking builds on ppc64le hosts.

The result of --host / --build is only used to detect if we are building for
Windows or not, which is why things are working on x86(-64) build hosts
without specifying --host even though we are cross compiling - But this may
change in the future, so we better pass the correct values.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>